Ac-RYYRWK-NH2 is a highly effective and specific partial agonist for the nociceptin receptor (NOP). It demonstrates a remarkable affinity for rat cortical membranes ORL1, with [3H]Ac-RYYRWK-NH2 exhibiting a Kd value of 0.071 nM. However, it shows negligible affinity towards μ-, κ-, or δ-opioid receptors.

Binding studies with [3H]ac-RYYRWK-NH2 on rat cortical membranes identified a single high-affinity binding site (Kd = 0.071 nM). Furthermore, Naloxone benzoylhydrazone competitively inhibited [3H]ac-RYYRWK-NH2 binding to rat cortical membranes (Ki = 104 nM) and to human ORL1 receptors (Ki = 136 nM). It also displaced [125I]Tyr14-NC-OH binding to ORL1 receptors with a Ki of 37 nM, indicating specificity as it showed no affinity for μ-, κ-, or δ-opioid receptors.